In this paper, a simple static var compensating scheme using a cascaded two-level inverter-based multilevel inverter is proposed. The topology consists of two standard two-level inverters connected in cascade through open-end windings of a three-phase transformer. The dc link voltages of the inverters are regulated at different levels to obtain four-level operation. The simulation study is carried out in MATLAB/SIMULINK to predict the performance of the proposed scheme under balanced and unbalanced supply-voltage conditions. A laboratory prototype is developed to validate the simulation results. The control scheme is implemented using the TMS320F28335 digital signal processor. Further, stability behavior of the topology is investigated. The dynamic model is developed and transfer functions are derived. The system behavior is analyzed for various operating conditions.

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Cascaded Two-Level Inverter-Based Multilevel
STATCOM for High-Power Applications
ABSTRACT
In this paper, a simple static var compensating scheme using a cascaded two-level inverter-based
multilevel inverter is proposed. The topology consists of two standard two-level inverters
connected in cascade through open-end windings of a three-phase transformer. The dc link
voltages of the inverters are regulated at different levels to obtain four-level operation. The
simulation study is carried out in MATLAB/SIMULINK to predict the performance of the
proposed scheme under balanced and unbalanced supply-voltage conditions. A laboratory
prototype is developed to validate the simulation results. The control scheme is implemented
using the TMS320F28335 digital signal processor. Further, stability behavior of the topology is
investigated. The dynamic model is developed and transfer functions are derived. The system
behavior is analyzed for various operating conditions.
KEYWORDS
1. DC-link voltage balance
2. Multilevel inverter
3. Power quality (PQ)
4. Static compensator (STATCOM)
SOFTWARE: MATLAB/SIMULINK

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BLOCK DIAGRAM
Fig. 1. Power systemand the STATCOM model.
EXPECTED SIMULATION RESULTS
Fig. 2. Frequency response ∆Vdc1(s) /∆δ1(s) at i’
q0 =1.02 p.u., δ1=-0.90,δ2=178.90,R1=
80 p.u., R2=60 p.u.

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Fig. 3. Root locus of the transfer function ∆Vdc1(s) /∆δ1(s) at i’
q0 = - 0.75 p.u., δ1=-0.570,δ2=179.60,R1=
80 p.u., R2=60 p.u.
Fig. 4. Reactive power control. (a) Source voltage and inverter current.
(b) DC-link voltages of two inverters.

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Fig. 5. Operation during fault. (a) Grid voltages on the LV side of the transformer. (b) -axis negative-sequence
current component i’
dn. (c) -axis negative- sequence current component i’
qn.
CONCLUSION
DC-link voltage balance is one of the major issues in cascaded inverter-based STATCOMs. In
this paper, a simple var

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Fig. 6. Experimental result: Capacitive mode of operation. (a) Source voltage (50 V/div) and STATCOM current (5
A/div). (b) DC-link voltages of inverter-1 and inverter-2 (20 V/div). Time scale: 5 ms/div. (c) Harmonic spectrumof
current.

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Fig. 7. Experimental result: Mode change from capacitive to inductive. (a) DC-link voltages of inverter-1 and
inverter-2 (20 V/div). Time scale: 100 ms/div. (b) Source voltage (100 V/div) and STATCOM current (5 A/div) in
steady state. Time scale: 100 ms/div.
compensating scheme is proposed for a cascaded two-level inverter- based multilevel inverter.
The scheme ensures regulation of dc-link voltages of inverters at asymmetrical levels and
reactive power compensation. The performance of the scheme is validated by simulation and
experimentations under balanced and unbalanced voltage conditions. Further, the cause for
instability when there is a change in reference current is investigated. The dynamic model is
developed and transfer functions are derived. System behavior is analyzed for various operating
conditions. From the analysis, it is inferred that the system is a non minimum phase type, that is,
poles of the transfer function always lie on the left half of the -plane. However, zeros shift to the
right half of the -plane for certain operating conditions. For such a system, oscillatory instability
for high controller gains exists.
REFERENCES
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[2] B. Singh, R. Saha, A. Chandra, and K. Al-Haddad, “Static synchronous compensators
(STATCOM): A review,” IET Power Electron., vol. 2, no. 4, pp. 297–324, 2009.
[3] H. Akagi, H. Fujita, S. Yonetani, and Y. Kondo, “A 6.6-kV transformerless STATCOM
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multilevel inverter and its application in shunt compensation of distribution systems,” IEEE
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[5] H. Akagi, S. Inoue, and T. Yoshii, “Control and performance of a transformerless cascaded
PWM STATCOM with star configuration,” IEEE Trans. Ind. Appl., vol. 43, no. 4, pp. 1041–
1049, Jul./Aug. 2007.